1. Field of the Invention
This invention relates to a method of creating an expanded region on a tube, and more specifically, to a method of using an inserted material to create an expanded region on cooling tubes for a catalytic combustor for a combustion turbine so that the cooling tubes maintain contact with one another and dampen vibration.
2. Background Information
Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator.
Typically, the combustor assembly creates a working gas at a temperature between 2,500 to 2,900 degrees Fahrenheit (1371 to 1593 degrees centigrade). At high temperatures, particularly above about 1,500 degrees centigrade, the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO2, collectively known as NOx. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the of air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.
Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature regions of non-well-mixed flames add to a flame""s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.
Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions, See e.g., U.S. Pat. No. 5,826,429. It is, therefore, desirable to have a combustor assembly that bums a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. As shown in U.S. patent application Ser. No. 09-670,035, which is incorporated by reference, this is accomplished by splitting the flow of compressed air through the combustor. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed.
One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. To reduce the temperature within the catalyst bed, prior art included a plurality of closely-oriented, parallel cooling tubes. These cooling tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. The inner surface of the cooling tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the cooling tubes. The fuel/air mixture passed over the tubes, and reacted with, the catalyst bed. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed. See U.S. patent application Ser. No. 09-670,035.
The disadvantage of such cooling systems was susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including but not limited to: degradation of connecting joints (e.g. brazing of the cooling conduits to the tubesheet); deformations due to tube to tube or tube to support structure impacting; and premature ignition, known as backflash, which results from irregular and reverse flow around and through the cooling conduits. Moreover, vibration of the cooling conduits or tubes, must be eliminated to prevent insufficient cooling, improper fuel reactions and even physical damage to the structural elements of the combustor.
Nonuniform tube expansion and overall tube expansion has been achieved by mechanical methods as propelling a ball through the overall tube length, pressing a pointed die in the end of tube to flare the end, and expanding a collet within the tube body. Each of these prior methods of tube expansion has its own shortcomings and none can achieve localized, uniform expansion. The collet approach is limited in that uniform expansion is not achieved and localized cracking of the tube wall may result. Pressing a pointed die in the end of the tube, if exactly centered, can produce a simple conical flare at the end of a tube but cannot achieve more complex shapes such as bulges. Propelling a ball through the tube has been successfully used in overall tube expansion but is ineffective in localized bulging or flaring of tubes.
None of the existing methods of tube expansion can achieve the localized and uniform tubular expansions at an intermediate portion of the tube necessary to suppress vibration of the parallel cooling conduits within a catalytic combustor.
There is, therefore, a need for an effective method of making uniform, localized expanded regions, or xe2x80x9cbulges,xe2x80x9d on the intermediate portions of a cooling tube for a catalytic reactor assembly of a combustion turbine.
There is further a need for a method of assembling the catalytic combustor so that the plurality of bulged cooling tubes contact one another thus suppressing vibration and minimizing degradation of the assembly.
These needs, and others, are met by the instant invention, which provides a method to create uniform localized expansions on the intermediate portion of a cooling tube. In turn, the tubes, whether assembled so that the expansion on one tube contacts the expansions on adjacent tubes, or so that the expansions on one tube are staggered with respect to the expansions on adjacent tubes thus contacting the unexpanded regions of that tube, create a dampening device by maintaining tube to tube contact and minimizing vibration.
The preferred method of expanding tubes utilizes a combination of localized softening of the tube by applying an annealing heat treatment followed by internal pressurization of a fluid to create an outward radial force. One way of providing such internal pressurization is hydraulically, by filling a tube with hydraulic fluid, sealing it, and then applying pressure using a pump. To avoid cracking the tube from work hardening, this technique may be repeated multiple times, reannealing the tube, and gradually applying greater pressure with each iteration until the desired bulge is formed. Work hardening is the phenomenon in which steel hardens due to cold working or working the steel when it is cool or unannealed. As the steel stretches and hardens it becomes more susceptible to cracking thus necessitating reheating or reannealing between internal pressurization steps. To further refine the process and add precision to the shape and size of the bulges, the tube may be placed in a rigid die having a machined cavity corresponding to the desired bulge.
Alternative hydraulic pressure methods may be employed to bulge the tube. One such method would be to immerse a portion of an annealed tube which has been filled with water and sealed, into a cryogenic liquid, such as liquid nitrogen, until the water freezes. As the water freezes, the fluid water is compressed, thereby increasing pressure in the tube. Also, if the tube remains in contact with the cryogenic liquid, ice may form within the annealed portion of the tube. As the water freezes and expands, the annealed portion of the tube is expanded. Expansion could also be achieved by other methods of internal pressurization, including but not limited to pneumatic pressurization and heat treatment of a solid insert with a higher coefficient of thermal expansion.
This method of forming expanded regions may also be performed after the tubes are attached to the tube sheet. That is, an intermediate portion of each tube is first given an annealing heat treatment and then the tubes are attached to the tube sheet as is known in the prior art, forming a tube sheet assembly. Each tube has one end plugged and is then filled with water. The other end of each tube is then plugged. One end of the tube sheet assembly is then dipped in a cryogenic fluid. As the water in the tube freezes, the annealed portion of each tube will bulge until it contacts an adjacent tube. Thus, because the tubes expand to each other, the size of each expansion does not need to be rigidly controlled.
It is an object of this invention to provide a method of forming at least one generally uniform, localized expansion on a cooling tube for a catalytic combustor.
It is further an object of this invention to provide a method of forming various expansion lengths, widths and heights on a cooling tube for a catalytic combustor.
A still further object of this invention is to provide a method of assembling a catalytic combustor assembly so that the cooling tubes, having an expanded region, contact one another, thus suppressing vibration.